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Rates of adsorption and desorption: Entropic contributions and errors due to mean-field approximations
1Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh 208016, India.
The Journal of Chemical Physics
|May 17, 2019
Summary
Exact classical rate calculations reveal that mean-field approximations can cause significant errors in adsorption and desorption rate constants. Sticking probability is coverage-dependent, impacting adsorption rates, while entropic contributions vary with temperature.
Area of Science:
- Chemical Kinetics
- Surface Science
- Computational Chemistry
Background:
- Accurate calculation of adsorption and desorption rates is crucial for understanding surface phenomena.
- Transition-state theory (TST) is a common method, but its approximations may limit accuracy.
- Real systems exhibit complex dynamics and entropic effects influencing reaction rates.
Purpose of the Study:
- To perform exact classical rate calculations for adsorption and desorption.
- To assess the accuracy of mean-field approximations in TST for these processes.
- To investigate the role of dynamical factors and entropic contributions on rate constants.
Main Methods:
- Exact classical rate calculations were employed.
- Desorption rates were computed using TST with a distant dividing surface.
- Adsorption rates incorporated a dynamical sticking factor.
- Entropic contributions were compared to 2D ideal gas and lattice gas models.
Main Results:
- Mean-field approximations in TST can lead to over two orders of magnitude error in desorption rate constants due to barrier fluctuations.
- Sticking probability, crucial for adsorption, is highly sensitive to surface coverage.
- Adsorption rates calculated with mean-field assumptions showed minimal deviation from exact rates.
- Entropic contributions to desorption align with the 2D ideal gas model at high temperatures but deviate significantly at lower temperatures.
Conclusions:
- Mean-field approximations are unreliable for desorption rate calculations, especially when large fluctuations occur.
- Adsorption dynamics, particularly sticking probability, are critical and coverage-dependent.
- The choice of adsorption model significantly impacts the accuracy of entropic contributions to desorption rates across different temperature ranges.
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